1. Field of the Invention
The present invention relates to a semiconductor device having a conductor usable for registration and a method for fabricating a semiconductor device.
2. Description of the Related Art
In recent, with the downsizing of electronic devices or the provision of higher functionalities for electronic devices, a CoC (Chip on Chip) semiconductor device has been developed, including a chip stacked body formed of a plurality of semiconductor chips stacked on each other. In the CoC semiconductor device, the registration of semiconductor chips, that is, the alignment of semiconductor chips is performed in stacking semiconductor chips on each other.
FIGS. 2B and 3 in JP2005-217071A disclose alignment marks for use in aligning semiconductor chips. A semiconductor chip described in JP2005-217071A is a chip for a multichip semiconductor device having two or more conductive through plugs. One or more plugs among a plurality of through plugs are used for alignment marks. In other words, the alignment mark is formed of the same conductive member as the conductive member of the through plug, a metal material, for example. This alignment mark can be identified on the front and back surface of the chip. Thus, the alignment mark can be fabricated by the same process steps as the process steps for the through plug.
The alignment mark described in JP2005-217071A is fabricated by the same process steps as the process steps for the through plug. Therefore, the alignment mark is made of the same metal material as the metal material of the through plug, copper, for example. The inventor found that the characteristics of a semiconductor element formed on a semiconductor substrate sometimes deteriorate when metal atoms forming an alignment mark diffuse into the semiconductor substrate.
For example, in the case where a semiconductor chip is a memory chip like a DRAM, electric charges stored in a capacitor as a semiconductor element tend to leak which is caused by metal impurity in a semiconductor substrate. Thus, the data hold time of the DRAM is shortened.
Therefore, in a semiconductor chip including alignment marks formed of a conductive member, it is desirable to suppress deterioration of device characteristics caused by diffusion of atoms that form the conductive member.